Text-book of comparative anatomy . cuticle; in Phoronis it is delicate,and the skin therefore secretes a detached chitinous envelope, whichserves as a dwelling tube. The Bryozoa generally form a rough hardcuticle (cell, ectocyst) whose aperture can be closed by a cover, andwhich often calcifies. In a similar way the mantle of the Brachiopodasecretes a bivalve shell which is generally calcareous, less frequentlyhorny. This shell of the Brachiopoda (Fig. 125) cannot be comparedwith the similarly bivalved shell of the Mussel (Lamellibranchiata,Cochlidce). The two shell valves of the former are do
Text-book of comparative anatomy . cuticle; in Phoronis it is delicate,and the skin therefore secretes a detached chitinous envelope, whichserves as a dwelling tube. The Bryozoa generally form a rough hardcuticle (cell, ectocyst) whose aperture can be closed by a cover, andwhich often calcifies. In a similar way the mantle of the Brachiopodasecretes a bivalve shell which is generally calcareous, less frequentlyhorny. This shell of the Brachiopoda (Fig. 125) cannot be comparedwith the similarly bivalved shell of the Mussel (Lamellibranchiata,Cochlidce). The two shell valves of the former are dorsal and ventral;each valve is symmetrical; the median plane of the body divideseach valve into two lateral congruent halves. In the mussels, onthe contrary, we distinguish a right and a left shell. The medianplane passes between the two shell valves. Each valve is asym-metrical. The gaping edge of the shell in the mussels is ventral, inthe Brachiopoda anterior; the closed edge where the two valves IV VERMES—THE INTEG UMEXT 191. B are joined by a hinge is dorsal in the mussels, in the Brachiopoda itis posterior. We have already, inthe systematic review, saidwhat is most important con-cerning the oral tentaclesand oral arms, which arevery characteristic of theProsopygia • the variouspositions of the anus werealso described. Referto it also for the outerform and organisation ofthe Piotatoria and CJuetog-natha.
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
